In general, as an automotive fuel, liquefied petroleum gas (LPG) is less expensive and has a clean exhaust gas in comparison to gasoline.
Also, an LPG engine may be driven at a high compression ratio without knocking due to a high octane number, may theoretically have a high output in comparison to a gasoline engine due to a high heating value per unit mass, may lose a small amount of lubricating oil, and may be warmed up fast.
Currently, due to the above advantages, the number of LPG vehicles is increasing and, particularly, liquefied petroleum injection (LPI) or liquid phase LPG injection (LPLI) systems for injecting LPG in a liquid phase to reduce an exhaust gas and to improve an output are popular. Despite the above advantages, due to a small number of LPG filling stations in comparison to gasoline filling stations, bi-fuel vehicles running on gasoline and LPG are demanded. Most bi-fuel vehicles use a combination of an LPG evaporating system and a gasoline injection system, and such vehicles are limited to satisfy tightened environmental regulations. Accordingly, current bi-fuel vehicles are more focused on a combination of an LPG liquefied injection system and a gasoline injection system.
In particular, these bi-fuel vehicles allow users to switch fuels between gasoline and LPG. Here, when a fuel switch signal is input, if a previous fuel is in a cut state, it is immediately switched to a replacing fuel. If the previous fuel is not in the cut state, a cylinder is counted until the previous fuel is completely injected, the injection of the previous fuel is stopped when the counting is completed, and injection is performed by adding a correction value to an injection amount of the replacing fuel, thereby switching fuels.
However, in the above fuel switching method, since a deviation of an air-fuel ratio may occur, an exhaust gas may be increased and an accidental fire may be caused.
Also, if fuels are switched, an abrupt output variation may occur (due to a difference in torque characteristics between fuels). The output variation may increase a mechanical stress on other components of a vehicle and thus a user may be under great pressure while driving the vehicle.
Furthermore, the output variation of an engine may cause inconvenience while driving a vehicle and may cause great and small traffic accidents.